The present invention generally relates to a fuel control system for an automobile internal combustion engine having an exhaust gas recirculating (EGR) system and an air-fuel mixture control. More particularly, the present invention relates to an automobile fuel control system for controlling in sequence the EGR system and the air-fuel mixture control in accordance with the engine temperature and in dependence on the composition of the exhaust gases emitted from the engine for the purpose of minimizing pollutants of the exhaust gases.
There is known an automobile fuel control system which comprises an exhaust composition sensor disposed on the exhaust system of the engine for sensing the oxygen concentration in exhaust gases emitted from the engine, and an air-fuel mixture control including a mixing ratio compensator, said air-fuel mixture control being operable in response to an output signal from the composition sensor for causing the compensator to adjust the mixing ratio of the combustible air-fuel mixture being supplied to the engine to a value substantially equal to the stoichiometric air-fuel mixing ratio required for a three-way catalytic converter on the exhaust system to work at its maximum conversion efficiency, that is, to minimize not only CO and HC components but also NOx components of the exhaust gases. An example of this is disclosed in, for example, the U.S. Pat. No. 4,088,095, patented on May 9, 1978. However, this patent is silent as to not only the use of a temperature sensor for sensing the temperature of the engine and the operative association of such temperature sensor with the air-fuel mixture control, but also the use of the EGR system in combination with the air-fuel mixture control.
The Japanese Laid-open Patent Publication No. 51-102721 laid open to public inspection on Sept. 10, 1976, discloses a similar automobile fuel control system utilizing the EGR control in combination with the air-fuel mixture control. According to this Japanese publication, the exhaust composition sensor is electrically connected to the air-fuel mixture control including an actuator through a difference detector. A temperature sensor for generating an output signal indicative of the engine temperature is electrically connected to the difference detector and also to the EGR control which includes a solenoid operated valve. The difference detector is so designed as to generate an output signal indicative of the difference between the output signal from the composition sensor, which is indicative of the oxygen concentration in the exhaust gases, and a reference signal representative of a predetermined air-fuel mixing ratio of the combustible air-fuel mixture.
In this automobile fuel control system disclosed in the Japanese publication, when the engine temperature is low, such as during the cold start of the engine, the EGR control is held in position to cause the solenoid operated valve to interrupt the circulation of a portion of the exhaust gases back to the fuel intake system of the engine on one hand and the air-fuel mixture control is held in an inoperative position. However, as the engine is warmed up, the air-fuel mixture control is brought into operation and, at the same time, the EGR control is operated to cause the solenoid operated valve to initiate the circulation of a portion of the exhaust gases back to the fuel intake system of the engine. In this way, at the time the air-fuel mixture control and the EGR control are simultaneously operated, the air-fuel mixture being supplied to the engine is considerably leaned temporarily for a certain period of time, resulting in a fluctuation of the power output of the engine and/or reduction of the drivability of an automobile.
On the other hand, where the EGR control is operated in delayed relation to, that is, subsequent to, the operation of the air-fuel mixture control, it has been found that, since during the warm-up of the engine, not only does the engine requires an enriched air-fuel mixture, but also the catalytic converter has not yet been warmed to a temperature required for the catalyst in the catalytic converter to work, the amount of the NOx components of the exhaust gases inhibited by the catalytic converter from being discharged to the atmosphere is smaller than that inhibited by the circulation of the portion of the exhaust gases back to the engine fuel intake system even though the air-fuel mixture control has been operated and, in addition, a prolonged period of time is required to complete the warm-up of the engine. In this case, since the enriched air-fuel mixture is determined by the mixing ratio control, the control itself requires a complicated structure and, therefore, is expensive.